Online shopping has trained consumers to look for reviews on the Web before purchasing anything, even if they plan on buying the item over the phone or at a retail outlet.

According to Internet-based research firm YouGov, however, the trust many consumers place in online reviews might be somewhat misplaced. A surprising 21 percent of Americans surveyed admitted to reviewing products they hadn't ever bought or tried. Their motives differed: A third of respondents offered the nebulous explanation that they just felt like it. Others said they posted a review on behalf of someone else (23 percent), because they didn't like the idea of the product (22 percent), or because they didn't like the manufacturer or service provider (19 percent). Ten percent admitted to posting spoof reviews.

In one of the most famous spoofs, 1,916 people to date have gone on Amazon to post reviews of the Bic Cristal for Her line of pens, but many of those are widely accepted to be fakes. Reviewers praise Bic, for example, for providing an alternative to the "masculine, fast-paced pens of today" and giving women a "pen they can use all month long." Jokes like these mirror reviews from those who didn't like the idea of the product, but they're unlikely to fool a consumer.

Nonetheless, the importance of reviews cannot be overstated; they are inextricably connected to the modern shopping experience. A full 93 percent of consumers report occasionally checking product reviews before making purchases; a quarter always check reviews, and half do it sometimes, according to the research.

Naturally, no company can ever expect every review to be positive. Among legitimate reviewers who left negative comments, most frequently did so for altruistic reasons: Eighty-eight percent did so to warn others about the product or service. Others did it for more personal reasons: Twenty-one percent hoped to receive refunds or help from the company; 23 percent did so to feel less angry about their experiences. "There's something cathartic about that," says YouGov researcher Anne Gammon. "But unlike a vent with a customer service agent over the phone, those one-star online reviews live on indefinitely."

The most popular place for people to leave their reviews is Amazon, used by 42 percent of consumers. Google (14 percent), Yahoo! (13 percent), and Yelp (10 percent) command a smaller share of the reviews.

Although the YouGov study did not examine whether the fake reviews skewed positively or negatively, both ends of the spectrum have implications for consumers and the companies being reviewed.

YouGov's survey was conducted just as a Virginia court decided in January that Yelp needed to disclose the IP addresses of customers who left negative reviews for Hadeed Carpet Cleaning, which the company argued were defamatory. The First Amendment rights of people who patronized the business are protected, according to the decision, but the same could not be said for those who have never patronized the business. The court decision opens the door for those who leave fake reviews to find themselves in legal hot water.

"The ruling itself puts more power back in SMBs' hands," Gammon says, "but it's bad for the consumer overall, because then the fear of retribution becomes a real thing."

Currently, just 7 percent of those who have never left reviews—good or bad—said they refrained from doing so out of a fear of retribution. Though that number might seem insignificant now, it could certainly grow as courts and legislators strip away some of the online anonymity that has emboldened people to express their unguarded opinions in the first place. In the United Kingdom, where Gammon has lived, for example, more people are being held accountable for their behavior on social media. Twitter users have been sued for libel or defamation for statements they made in their posts.

While much of the attention has focused on bogus negative reviews, fake reviews that are positive are also a concern, especially as brands are starting to incorporate consumer comments into their marketing efforts. Puerto Rico Tourism, for example, recently launched an advertising campaign that features actors reading real Trip Advisor reviews praising the island's attractions. It also prominently features them on its Web site. This is regarded as an about-face, given that many brands have shied away from reviews in the past because of their unpredictability, a sentiment YouGov's research now indicates was well-founded.